Re: [PATCH] mm/page_alloc: do bulk array bounds check after checking populated elements

[Mel Gorman]

On Mon, Jun 28, 2021 at 12:27:59AM -0400, Dave Jones wrote:
> On Fri, Jun 18, 2021 at 01:51:02PM +0100, Mel Gorman wrote:
>  > Dan Carpenter reported the following
>  > 
>  >   The patch 0f87d9d30f21: "mm/page_alloc: add an array-based interface
>  >   to the bulk page allocator" from Apr 29, 2021, leads to the following
>  >   static checker warning:
>  > 
>  >         mm/page_alloc.c:5338 __alloc_pages_bulk()
>  >         warn: potentially one past the end of array 'page_array[nr_populated]'
>  > 
>  > The problem can occur if an array is passed in that is fully populated. That
>  > potentially ends up allocating a single page and storing it past the end of
>  > the array. This patch returns 0 if the array is fully populated.
>  > 
>  > Fixes: 0f87d9d30f21 ("mm/page_alloc: add an array-based interface to the bulk page allocator")
>  > Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
>  > Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxxx>
>  > ---
>  >  mm/page_alloc.c | 4 ++++
>  >  1 file changed, 4 insertions(+)
>  > 
>  > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>  > index 7124bb00219d..ef2265f86b91 100644
>  > --- a/mm/page_alloc.c
>  > +++ b/mm/page_alloc.c
>  > @@ -5056,6 +5056,10 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
>  >  	while (page_array && nr_populated < nr_pages && page_array[nr_populated])
>  >  		nr_populated++;
>  >  
>  > +	/* Already populated array? */
>  > +	if (unlikely(page_array && nr_pages - nr_populated == 0))
>  > +		return 0;
>  > +
>  >  	/* Use the single page allocator for one page. */
>  >  	if (nr_pages - nr_populated == 1)
>  >  		goto failed;
> 
> 
> This made it into 5.13 final, and completely breaks NFSD for me (Serving tcp v3 mounts).
> Existing mounts on clients hang, as do new mounts from new clients.
> Rebooting the server back to rc7 everything recovers.  Bisect lands on
> this commit.
> 

Thanks Dave, can you try this?

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ef2265f86b91..04220581579c 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5058,7 +5058,7 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
 
 	/* Already populated array? */
 	if (unlikely(page_array && nr_pages - nr_populated == 0))
-		return 0;
+		return nr_populated;
 
 	/* Use the single page allocator for one page. */
 	if (nr_pages - nr_populated == 1)